feat(app): first-run data location wizard step

On first launch (no data yet, no portable dir, no pointer file),
the wizard now shows a new step-0 screen asking where to store
wallets, config and certificates.

AppPaths changes:
- DefaultDataRoot() → ~/.PalladiumWallet (home, always writable)
- IsDataLocationConfigured() → true when portable / override / pointer
  already written / legacy or default already has data
- ConfigureDataLocation(root) writes a bootstrap pointer file and
  creates the directory
- DataRoot() resolution order: override → portable → pointer → legacy
  (has data) → default

ViewModel: new StepDataLocation step, UseDefaultDataLocationCommand,
ApplyDataLocation(root) (also called from View's folder picker).

View: new wizard panel with description, default-path display, two
buttons (use default / choose folder); folder picker via
StorageProvider.OpenFolderPickerAsync.

Loc: add wiz.data.* keys (6 languages); fix fallback language "it"→"en";
update test assertion accordingly.

tests/PalladiumWallet.Tests/Wallet/TransactionFactoryTests.cs

@@ -197,7 +197,7 @@ public class TransactionFactoryTests
         {
             File.WriteAllText(path, "{ rotto ");
             var loaded = PalladiumWallet.Core.Storage.AppConfig.Load(path);
-            Assert.Equal("it", loaded.Language);
+            Assert.Equal("en", loaded.Language);
             Assert.Equal("PLM", loaded.Unit);
         }
         finally